Hydrocarbon combustion in diesel engines, stationary gas turbines, and other systems generates exhaust gas that must be treated to remove nitrogen oxides (NOx), which comprises NO (nitric oxide) and NO2 (nitrogen dioxide), with NO being the majority of the NOx formed. NOx is known to cause a number of health issues in people as well as causing a number of detrimental environmental effects including the formation of smog and acid rain. To mitigate both the human and environmental impact from NOx in exhaust gas, it is desirable to eliminate these undesirable components, preferably by a process that does not generate other noxious or toxic substances.
Exhaust gas generated in lean-burn and diesel engines is generally oxidative. NOx needs to be reduced selectively with a catalyst and a reductant in a process known as selective catalytic reduction (SCR) that converts NOx into elemental nitrogen (N2) and water. In an SCR process, a gaseous reductant, typically anhydrous ammonia, aqueous ammonia, or urea, is added to an exhaust gas stream prior to the exhaust gas contacting the catalyst. The reductant is absorbed onto the catalyst and the NOx is reduced as the gases pass through or over the catalyzed substrate. In order to maximize the conversion of NOx, it is often necessary to add more than a stoichiometric amount of ammonia to the gas stream. However, release of the excess ammonia into the atmosphere would be detrimental to the health of people and to the environment. In addition, ammonia is caustic, especially in its aqueous form. Condensation of ammonia and water in regions of the exhaust line downstream of the exhaust catalysts can result in a corrosive mixture that can damage the exhaust system. Therefore the release of ammonia in exhaust gas should be eliminated. In many conventional exhaust systems, an ammonia oxidation catalyst (also known as an ammonia slip catalyst or “ASC”) is installed downstream of the SCR catalyst to remove ammonia from the exhaust gas by converting it to nitrogen. The use of ammonia slip catalysts can allow for NOx conversions of greater than 90% over a typical diesel driving cycle.
It would be desirable to have a catalyst that provides for both NOx removal by SCR and for selective ammonia conversion to nitrogen, where ammonia conversion occurs over a wide range of temperatures in a vehicle's driving cycle, and minimal nitrogen oxide and nitrous oxide by-products are formed.